An aerosol is a suspension of particles in a gaseous medium. The particles are formed by the action of winds (producing dust and sea-salt), by combustion (producing organics and black carbon), by biological and chemical processes (the oxidation of gases and chemical reactions that lead to particle formation) and by human activity (haze pollution in the form of black carbon, sulfates, nitrates, and organics). Once in the atmosphere, particles can impact the earth’s climate directly through the absorption and scattering of sunlight. They also impact climate indirectly by acting as nuclei for the formation of cloud droplets and ice crystals. The photo shows dust from the Sahara interacting with cumulus clouds in the Caribbean (photo courtesy Darin Toohey).